I have seen a switch from 3G to 1X occasionally on the DX (especially in low signal areas-but not always), but far less than I did on my Blackberry Storm 1 and 8703e. So I would say that it is not inherent in the design.
Each phone number hashes to a specific control channel so it is very possible that two phones can behave differently in the same location, especially if the issue is isolated to one particular location.
Happens to me all the time, I think its software related. Hopefully the froyo update fixes it. Mine I have to turn on airplane mode and off and it goes back to 3g. Also It usually turns off once every 2 or 3 days. Today It was stuck on a roaming network in a good coverage area. I had to dial *22800 option 2(i work for vzw tech) and power cycle., that fixed it. But the 3g to 1x thing is annoying. I heard froyo fixes all those bugs. Overall its still a strong phone and does what it needs to do.
I'm sure it's an issue of weak radio hardware. I had HTC Droid Incredible and it was steadily on 3G, now I have Motorola Droid 3 dropping 3G and switch to 1x within same areas. Moreover Motorola is loosing network swtiching between EV-DO and 1x. At such a moment phone is unreachble and could even reboot.